Arc detector for waveguide system



June 22, 1965 F. P. SAVALLI ARC DETECTOR FOR WAVEGUIDE SYSTEM Filed April 27, 1961 PULSE LvloouLATon MAGNETRON .L MODULATOR 21 INTERLOCK =5 2 27 cmcuw Is+ 33 c THYRATRON r .c. 39 o AMPLIFIER INVENTOR. FRANK P SAv/JLL/ BY ATTORNEY 7 United States Patent 3,191,046 ARC DETECTOR FUR WAVEGUHDE SYSTEM Frank P. Savalli, Largo, Fla, assignor to Sperry Rand (Importation, Great Neck, N.Y., a corporation of Delaware Filed Apr. 27, 1961, Ser. No. 1%,975 6 Claims. Cl. 250-217) This invention relates to improvements in protective apparatus for microwave power systems, and more particularly to devices for detecting the presence of arcing in hollow conductive wall waveguides.

Power breakdown in waveguides can occur as a result of an excessive voltage gradient at some point where there is a discontinuity in the wall surface such as a minor imperfection, or at a junction where the coupling flanges do not meet properly, for example. In any event, such breakdown is accompanied by arcing, i.e. an electrical discharge producing visible light, and by a change in the transmission characteristics of the waveguide which reflects a corresponding change in the loading on the microwave power source. The breakdown may be progressive, the are being small at first and then rapidly becoming larger as the air or other gas in the vicinity becomes ionized. If the resulting load change becomes large enough, the power source, e.g. an expensive magnetron or klystron, will be damaged.

One method of breakdown detection involvesthe use of a reflectometer placed in the waveguide circuit. Reflections due to arcing produce a signal which may be used to shut down the system. This arrangement is not entirely satisfactory because it responds to reflections or variations in reflection that may occur in normal operation and would not damage the equipment, resulting in false alarms or unnecessary shutdowns. Although the reflectometer system is thus too sensitive to reflections, it is at the same time not sensitive enough to detect small arcs that may precede a damaging breakdown but do not change the reflection very much before such breakdown actually occurs.

Another method, upon which the present invention is an improvement, depends upon the detection of light produced by arcing inside the waveguide. This has been done with rectangular waveguides by providing an H plane bend in the wave guide, with a transparent window in the outside wall of the guide at the center of the bend. A photoelectric cell adjacent the window will see light in either direction from the bend and provide the warning orshut down signal. This arrangement has the disadvantage that the waveguide bend must be placed in the system arbitrarily where it may not be needed for any other reason, and may indeed be undesirable.

According to this invention a short straight section of waveguide is provided with an aperture through whicha small short rod or probe of transparent material passes. The end of the rod inside the guide is shaped to collect light traveling from either direction in the guide, for conduction to a photocell adjacent the other end of the rod, outside the guide. The short Waveguide section may be placed at any desired point in an existing waveguide system without changing the mechanical layout or the electrical characteristics. If desired, the arc detector section may be simply a part of the waveguide system provided ainters Patented June 22, int-i5 with a small aperture and with the arc detector head attached to the guide at that point.

The principal object of this invention is to provide arc detecting means of such sensitivity as to enable breakdown warning or automatic shutdown of a microwave system before the power source can be damaged.

Another object is to provide protective apparatus for microwave systems responsive directly to arcing but not responsive to variations in system operation such as refiections unaccompanied by arcing.

Another object is to provide an arc detector sensor device which is adapted to be placed conveniently between serially arranged waveguide elements of a microwave system without disturbing the transmission characteristics, and which may be sealed to maintain waveguide pressurization.

A further object is to provide a waveguide power breakdown detector which is itself capable of withstanding high power levels without breakdown.

Another major object of the invention is to provide a light-responsive arc detector device which does not require any waveguide bends, special terminations or other complex means to convey light from remote arcs to the photocell or other light sensitive element.

The invention will be described with reference to the accompanying drawing, wherein:

FIGURE 1 is a perspective view of an arc detector head, with portions broken away to show interior details of construction, and

FlGURE 2 is a schematic diagram of a microwave power transmission system including arc detector apparatus embodying the present invention.

Referring to FIGURE 1, a section 1 of hollow conductive waveguide is provided with an aperture 3 through which extends a rod 5 of transparent material such as methyl methacrylate. The waveguide may be of conventional rectangular cross section, in which case it is preferable to locate the aperture 3 in one of the narrow walls about midway between the upper and lower broad walls, as shown. The inner end of the rod 5 extends into the interior of the waveguide a short distance, preferably about equal to the diameter of the rod, and is provided with a wedge shaped tip 6. Preferably the dihedral angle is about 60 degrees. The apex line is disposed vertically as shown in FIGURE 1, so the planar surfaces of the wedge face generally axially of the waveguide. Rod 5 should extend into the interior ofwaveguide section 1 by a distance substantially less than one quarter guide wavelength, and the diameter of aperture 3 should be' substantially less than one quarter guide wavelength in order to assure that they produce substantially no perturbation in the Wave transmission characteristics of the waveguide.

A block 7 of dielectric material is arranged with a surface adjacent and contiguous with the wall of the waveguide 1 in an area surrounding the rod 5. The block 7 is provided with a bore adapted to receive and support the rod 5, which may have a shoulder for engagement with a conforming step in the bore. A continuation of the bore supports a photoelectric cell 11 with its window adjacent the outside end 13 of the rod 5.

The above described assembly is contained by a lighttight housing 15 secured to the waveguide 1. The terminals of the photoelectric cell 11 are connected by way of a conventional plug and socket arrangement 17 to a flexible two conductor cable 19.

Referring to FIGURE 2, the arc detector head of FIGURE 1 is designated by the reference numeral 2, and is shown connected between two serially disposed lengths of waveguide 21 and 23 forming parts of a microwave power transmission system between a magnetron 25 and a load device 27. Although the guides 21 and 23 are shown as straight, they may and ordinarily would include one or more bends, twists or other departures from Simple rectilinearity. The are detector head 2 may be provided with conventional waveguide coupling flanges mating with similar flanges on waveguides 21 and 23.

The magnetron 25 is arranged in known manner to be pulsed, i.e. periodically energized by high voltage pulses, produced .by a modulator 2?. The modulator 29 may be of conventional design and provided, as is usual in such devices, with an interlock circuit which, when opened, for example by opening of an access door, stops operation of the modulator. The interlock circuit is brought out to include the normally closed contacts 31 of a relay 33. If desired, the relay 33 may include an additional pair of normally open contacts (no-t shown) which may be used to actuate an indicator or alarm device when the V relay is energized.

The output terminals of the arc detector head 2 are conneoted to the input terminals of a D.-C. amplifier 35. The amplifier 35 is of conventional design, but a portion of its circuit is shown to illustrate the connection of an adjustable resistor 37 between the 3+ plate voltage supply terminal and the control grid of the first stage tube of the amplifier. The resistor 37, together with the grid leak resistor 39, forms a voltage divider network placing an adjustable positive (with respect to ground) potential on the grid of the first amplifier tube and also on the anode of the photoelectric cell 11. The'cathode of the first amplifier tube is biased positively with respect to ground potential in accordance with customary practice so that the grid is somewhat negative with respect to the cathode. Adjustment of the resistor 37 varies both the grid to cathode bias onthe amplifier and the polarizing bias on the photoelectric cell in the same sense, for simultaneous control of the amplifier gain and the photo cell sensitivity.

The output terminals of the amplifier 35 are connected to the control input terminals of a thyratron 41, the positive output terminal being connected to the thyratron control grid. The anode of the thyratron is connected to the energizing coil of the relay 3-3, and through a normally closed reset switch 43 to the 3-]- plate voltage supply terminal.

In the operation of the system, ambient light is excluded by the waveguide walls and the housing 15. The sensitivity control 37 may be set to a point just below that at which the normal dark current of the photocell ii. is enough to cause the output of the amplifier 35 to tire the thyratron 41. Once the thyratron is fired, it continues to conduct and maintain the relay 33 energized until the reset switch 43 is opened momentarily.

With the modulator 29 in operation, the magnetron 25 produces pulses of microwave energy which are transmitted through waveguides 21 and 23 and the are detector head 2 to the load 27. In the event that any arcing occurs within the guides, some of the resultant light travels directly and/ or by reflection from the interior surfaces of the waveguide walls to the tip 6 (FIGURE 1) of the rod 5, and through the rod 5 to the photocell =11. A very small amount of such light is enough to produce suflicien-t increase in the current through the photocell to cause the output of the amplifier 35 to fire the thyratron 41. Firing of the thyratron energizes the relay 33, opening contacts. 31 and stopping the modulator 29, which stops operation of the magnetron 25.

It has been found in practice that the described pro tective system will respond to arcs at points separated from the arc detector head by two waveguide bends, and yet will not cause false alarms or unnecessary shut downs. The system is particularly useful in breakdown test setups wherein production microwave devices are exposed to high power levels with the expectation that a certain percentage of the devices being tested will break down. In such arrangements the power source is usually short-lived due to the frequent overloads presented to it. With the described protective system, the power tube used in this service have exhibited useful lives in excess of the expectancy under normal operating conditions.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of'description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. Apparatus for detecting arcs in a waveguide system, comprising a waveguide section having a conductive wall provided with an aperture, an optically transparent rod extending from the exterior of said waveguide section'through said aperture and into the interior of said waveguide section, the portion of said rod within said waveguide section having a light collecting surface that has at least a portion thereof inclined with respect to the longitudinal axis of the waveguide section to collect light traveling within said waveguide and redirect it transversely through said aperture, the size of said aperture and the projection of said rod into said interior being such as to produce substantially inappreciable perturbation in the wave transmission characteristics of said waveguide section, and a photoelectric cell adjacent the end of said transparent rod outside said waveguide section and positioned to receive said transversely directed light.

2. The invention set forth in claim 1, wherein the end of said rod inside said waveguide section is provided with surfaces facing in generally opposite directions longi- .tudinally of said waveguide section to gather light from either direction within said waveguide section and adjacent waveguide sections connected thereto.

3. The invention set forth in claim 2, wherein the end of said rod inside said waveguide section is wedge shaped.

4. Apparatus for detecting arcs in a waveguide system, comprising a short straight section of rectangular wave guide provided with coupling flanges and adapted to be inserted between the proximate ends of two serially disposed lengths of waveguide in said system, said waveguide seotion being provided with an aperture in one of its narrow walls, the largest dimension of said aperture being substantially less than one quarter the guide wavelength of the highest frequency energy to be carried by said waveguide system, an optically transparent rod extending from the exterior of said waveguide section through said aperture and into the interior of said waveguide section a distance substantially less than said one quarter guide wavelength, whereby said aperture and said rod produce substantially no perturbation in the wave transmission characteristics of; said waveguide section, the portion of said rod within said waveguide section having a surfiace generally facing in each longitudinal direction within said waveguide and inclined with respect to the longitudinal axis thereof to gather light from opposite directions along the axis of said waveguide and to re direct the gathered light transversely through said aperture, a photoelectric cell, means for supporting said cell outside of said waveguide adjacent the end of saidrod in a manner to receive said transversely directed gathered light, an enclosure shielding said cell from ambient light, means for electrically connecting said cell to a device responsive to the output of said cell to indicate the presence of light inside said waveguide section.

5. Apparatus for detecting arcs in a waveguide systern comprising a section of short, straight waveguide having [a wall provided with an aperture, an optically transparent rod extending from the exterior of said waveguide section through said aperture and terminating within the interior of said waveguide section, the end of said rod within said waveguide having a surface generally facing in each longitudinal direction within said waveguide section and inclined with respect to the longitudinal axis thereof to redirect through said rod in a transverse direction the intercepted light traveling longitudinally along the waveguide, and a photoelectric cell adjacent said rod outside said waveguide section and positioned to receive said transversely directed light traveling through said rod.

6. The combination claimed in claim 5 wherein the largest dimension of said aperture and the extent of the rod within the waveguide section both are substantially less than one quarter guide wavelength.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner.

SAMUEL BERNSTEIN, Examiner. 

1. APPARATUS FOR DETECTING ARCS IN A WAVEGUIDE SYSTEM, COMPRISING A WAVEGUIDE SECTION HAVING A CONDUCTIVE WALL PROVIDED WITH AN APERTURE, AN OPTICALLY TRANSPARENT ROD EXTENDING FROM THE EXTERIOR OF SAID WAVEGUIDE SECTION THROUGH SAID APERTURE AND INTO THE INTERIOR OF SAID WAVEGUIDE SECTION, THE PORTION OF SAID ROD WITHIN SAID WAVEGUIDE SECTION HAVING A LIGHT COLLECTING SURFACE THAT HAS AT LEAST A PORTION THEREOF INCLINED WITH RESPECT TO THE LONGITUDINAL AXIS OF THE WAVEGUIDE SECTION TO COLLECT LIGHT TRAVELING WITHIN SAID WAVEGUIDE AND REDIRECT IT TRANSVERSELY THROUGH SAID APERTURE, THE SIZE OF SAID APERTURE AND THE PROJECTION OF SAID ROD INTO SAID INTERIOR BEING SUCH AS TO PRODUCE SUBSTANTIALLY INAPPRECIABLE PERTURBATION IN THE WAVE TRANSMISSION CHARACTERISTICS OF SAID WAVEGUIDE SECTION, AND A PHOTOELECTRIC CELL ADJACENT THE END OF SAID TRANSPARENT ROD OUTSIDE SAID WAVEGUIDE SECTION AND POSITIONED TO RECEIVE SAID TRANSVERSELY DIRECTED LIGHT. 